It is the usual practice to provide cooling fans in direct current power supplies. Such cooling fans are usually of the direct current type, and are energized directly by the power supply. Direct current fan motors have a tendency to draw surges of current which occur roughly at twice the RPM of the fan. If such current surges appear in the output of the power supply, they can create noise signals within the audio band. These noise signals become troublesome, especially when the power supplies are used to energize telemetry systems, and the like.
Accordingly, it is an objective of the present invention to provide an isolation circuit which effectively provides a constant current source for the fan motor, and effectively prevents the fan motor from introducing noise signals into the output of the power supply.